In general, a turbomolecular pump comprises a rotor and a stator having a plurality of stages, the rotor being carried by bearings. Going from the inside of the enclosure towards the outside, the gas pressure increases progressively from one stage to the next, and the stages close to the inside of the enclosure are considered to be low-pressure stages, while the stages close to the outlet are considered to be high-pressure stages.
When it includes magnetic or gas bearings for supporting the rotor, such a turbomolecular pump has the characteristic that its rotor is isolated physically and therefore electrically from the stator and from the body of the pump which finds itself at reference potential, i.e. at the ground potential of the equipment.
Turbomolecular pumps are frequently used in plasma deposition or etching equipment in the semiconductor industry.
While such turbomolecular pumps are being used in plasma deposition or etching processes, it has been observed that deposits tend to form of materials coming from the reaction products. For example, in a turbomolecular pump used in a plasma etching machine for etching semiconductor materials, residues coming from the etching of the resin masks tend to deposit on the inside surfaces of the rotor and of the stator, and to do so preferentially in the high-pressure stages of the turbomolecular pump.
In addition, in plasma deposition or etching methods, the turbomolecular pump and in particular its rotor are in direct contact with the plasma. As a result, the rotor, which is electrically isolated, is taken to a potential that is different from ground potential.
The pressure conditions in the high-pressure portion of the turbomolecular pump, combined with the short distance between the rotor and the stator and also with the potential difference between the rotor and the stator cause electrical discharges to occur between the rotor and the stator. In the absence of deposits, such discharges are distributed uniformly between the surfaces of the rotor and of the stator, and they do not do any damage.
Unfortunately, when deposits form, they disturb the discharges by creating preferential paths, and zones are created in which arc conditions occur in which high current densities are set up, thereby rapidly damaging the rotor.
In the prior art, attempts have already been made to remedy this problem in various ways.
In a first solution, a grid connected to ground has been interposed between the turbomolecular pump and the plasma in the enclosure. Unfortunately, the high density of the plasmas used requires very fine-mesh grids, sometimes of mesh size less than 100 .mu.m. Under such conditions, the presence of grids reduces the conductance of the pump considerably, and significantly reduces its pumping speed. In addition, such a very fine-mesh grid is a site on which deposits form that can then generate particles detrimental to the industrial process that is performed in the enclosure.
In another solution, attempts have been made to prevent deposits from forming by increasing the temperature of the turbomolecular pump so as to avoid deposition by condensation. However, given the nature of the materials used and the high speed of rotation of the rotor, temperatures are rapidly reached that generate phenomena of material creep, thereby destroying the pump without even being effective in preventing deposits from forming.